The present invention relates to semiconductor processing methods, and more particularly, to methods for making semiconductors materials in a form suitable for fabrication of thin-film transistor (“TFT”) devices.
Flat panel displays and other display units are used as visual imaging interfaces for the common and ubiquitous electronic devices and appliances such as computers, image sensors, and television sets. The displays are fabricated, for example, from thin films of liquid crystal and semiconductor material placed on glass or plastic substrates. Each display is composed of a grid (or matrix) of picture elements (“pixels”) in the liquid crystal layer. Thousands or millions of these pixels together create an image on the display. TFT devices fabricated in the semiconductor material layer are used as switches to individually turn each pixel “on” (light) or “off” (dark). The semiconductor materials used for making the TFTs, traditionally, are amorphous or polycrystalline silicon thin films. These films are deposited on to the substrates by physical or chemical processes at relatively low deposition temperatures in consideration of the low melting temperatures of the substrate materials used (e.g., glass or plastic). The relatively low deposition temperatures degrade the crystallinity of the deposited silicon films and cause them to be amorphous or polycrystalline.
Unfortunately, the device characteristics of a TFT fabricated in a silicon thin film undesirably degrade generally in proportion to the non-crystallinity of the silicon thin film. For industrial TFT device applications, silicon thin films of good crystalline quality are desirable. The crystallinity of a thin film of silicon deposited at low temperatures on a substrate may be advantageously improved by laser annealing. Maegawa et al. U.S. Pat. No. 5,766,989, for example, describes the use of excimer laser annealing (“ELA”) to process amorphous silicon thin films deposited at low temperatures into polycrystalline silicon thin films for LCD applications. The conventional ELA processes, however, are not entirely satisfactory at least in part because the grain sizes in the annealed films are not sufficiently uniform for industrial use. The non-uniformity of grain size in the annealed films is related to the beam shape of the laser beam, which is used in the ELA process to scan the thin film.
Im et al. U.S. Pat. No. 6,573,531 and Im U.S. Pat. No. 6,322,625 (hereinafter “the '531 patent” and “the '625 patent”, respectively), both of which are incorporated by reference herein in their entireties, describe laser annealing apparatus and improved processes for making large grained polycrystalline or single crystal silicon structures. The laser annealing processes described in these patents involve controlled resolidification of target portions of a thin film that are melted by laser beam irradiation. The thin film may be a metal or semiconductor material (e.g., silicon). The fluence of a set of laser beam pulses incident on the silicon thin film is modulated to control the extent of melting of a target portion of a silicon thin film. Then, between the incident laser beam pulses, the position of the target portion is shifted by slight physical translation of the subject silicon thin film to encourage epitaxial lateral solidification. This so-called lateral solidification process advantageously propagates the crystal structure of the initially molten target portion into grains of large size. The apparatus used for the processing includes an excimer laser, beam fluence modulators, beam focusing optics, patterning masks, and a motorized translation stage for moving the subject thin film between or during the laser beam irradiation. (See e.g., the '531 patent, FIG. 1, which is reproduced herein).
Consideration is now being given to ways of further improving laser annealing processes for semiconductor thin films, and in particular for recrystallization of thin films. Attention is directed towards apparatus and process techniques, with a view to both improve the annealing process, and to increase apparatus throughput for use, for example, in production of flat panel displays.